This is a 371 national phase application of PCT/IB2011/000926 filed 02 May 2011, claiming priority to Japanese Patent Application No. 2010-107381 filed 07 May 2010, the contents of which are incorporated herein by reference.
1. Field of the Invention
The invention relates to a technique for a fuel cell system that detects the amount of liquid water in a fuel cell.
2. Description of the Related Art
A fuel cell is, for example, formed so that a membrane electrode assembly formed by sandwiching both surfaces of an electrolyte membrane with an anode electrode and a cathode electrode is further sandwiched with a pair of separators to form a single cell and then a plurality of the single cells are stacked on top of each other. Fuel gas containing hydrogen is introduced into the anode electrodes of the fuel cell and oxidant gas, such as air (oxygen), is introduced into the cathode electrodes of the fuel cell to generate electricity by the electrochemical reaction between hydrogen and oxygen to thereby produce water.
Incidentally, during operation of the fuel cell, produced water is primarily produced in the cathode electrodes, and water content in each cathode electrode migrates to the anode electrode via the electrolyte membrane interposed between the cathode electrode and the anode electrode. Therefore, liquid water remains in the fuel cell at the time when power generation of the fuel cell is stopped, and, when power generation is stopped while leaving the remaining liquid water, water may freeze to damage the electrolyte membranes, or the like, in usage under a low-temperature environment, such as in cold climate areas and in winter. Then, purge gas is introduced into the anode electrodes and the cathode electrodes to perform purge process at the time when power generation of the fuel cell is stopped.
In a fuel cell system according to a related art, purge process is performed at a predetermined constant set amount at the time when power generation is stopped, so purge is sometimes performed for an unnecessarily long period of time. This may, for example, degrade the electrolyte membranes to decrease the performance of power generation.
Purge is performed after power generation is stopped and it is necessary to use energy stored in an electrical storage device, so, when purge process is carried out for an unnecessarily long period of time as described above, extra electric power in the electrical storage device is consumed and, therefore, for example, required electric power may run short at the time of a subsequent start-up of the fuel cell.
Then, it is desirable that the amount of liquid water in the fuel cell is estimated and then the purge process for the fuel cell is carried out on the basis of the estimated amount of liquid water, so a technique for estimating the amount of liquid water in the fuel cell is also suggested in the related art. For example, Japanese Patent Application Publication No. 2007-35389 (JP-A-2007-35389) describes a technique for estimating the amount of liquid water in the fuel cell on the basis of an accumulated value of generated current.
In addition, for example, Japanese Patent Application Publication No. 2006-338921 (JP-A-2006-338921) describes a technique for estimating the amount of liquid water in the fuel cell on the basis of a voltage difference in the fuel cell.
In addition, for example, Japanese Patent Application Publication No. 2006-278168 (JP-A-2006-278168) describes a technique for estimating the amount of liquid water in the fuel cell on the basis of a humidity and a pressure loss in the fuel cell and a technique for estimating the amount of liquid water in the fuel cell on the basis of the amount of produced water, a fuel cell temperature, an outside air temperature, a fuel cell load and an operation history.
In addition, for example, Japanese Patent Application Publication No. 2005-108673 (JP-A-2005-108673) describes a technique for estimating the degree of flooding by statistically processing the voltages of the fuel cell.
In addition, for example, Japanese Patent Application Publication No. 7-235324 (JP-A-7-235324) describes a technique for estimating the degree of flooding on the basis of the impedance of the fuel cell.
However, in the methods according to the above related arts, the voltage, humidity, pressure loss, or the like, of the fuel cell is poor sensitive, and multiple errors cannot be corrected using an operation history, or the like, so it is difficult to accurately estimate the amount of liquid water in the fuel cell.